There are numerous types of telephone line feed circuits to accommodate a number of different criteria as well as to satisfy or focus on various constraints peculiar to particular situations. Of significant interest to substantially every type of telephone line feed circuit is the capability of providing two-way voice communications while maintaining a substantially large trans-hybrid loss, adequate return loss and proper impedance matching without loss of signal strength over a band of desired frequencies. Other concerns involve designs that adequately protect the telephone line feed circuit against damage due to adverse voltages, such as lightning strikes to the line.
A number of telephone line feed circuits are entirely transformerless, and include only resistors, capacitors and solid state components. While this type of feed circuit can be characterized by a small size, they require rather closely-matched resistors and other components, thereby making the circuit very costly. Also, the power consumption of the transformerless type of hybrid can be quite substantial. Another limitation inherent in transformerless line feed circuits is the inability to operate completely between the power supply rails, e.g. a headroom limitation. Typically, transformerless solid state line feed circuits have a voltage operating range of about 8 volts less than the difference between the power supply rails. This reduced range of operating voltage limits the use of such type of line feed circuits to shorter loop lengths, as compared to line feed circuits that utilize transformers. Also, transformerless line feed circuits have a shortcoming in that they cannot sink telephone line current.
In telephone line feed circuits employing transformers, the designs can be less costly, but generally at the expense of the size of components. With larger components, this reduces the number of line feed circuits that can be assembled on a printed circuit board of reasonable size. The typical line transformer is a 1:1 transformer which must maintain a magnetizing inductance greater than 1.5 Henry while carrying 50mA in its primary winding. The physical dimensions are typically 3.4cm .times.3.2cm .times.1.4cm (1.33 inch .times.1.25 inch .times.0.55 inch).
As an apparent attempt to overcome the transformer size problem, U.S. Pat. No. 4,982,426 by Jakab discloses a two-transformer telecommunication line interface circuit that utilizes a small transformer wound with a resistance wire, and with the secondary thereof driven by a low impedance amplifier. It is difficult to realize exactly the required termination impedance, for example, 900 Ohm, in the resistance windings. Thus, the resistance of the resistance windings is made at a convenient value less than the 900 Ohm. External resistors are then used to pad out the total series resistance to exactly 900 Ohms. By utilizing a majority of the terminating resistance in the windings of the transformer, excessive power is dissipated in the transformer when subjected to over voltage conditions, resulting in failure of transformer. While it is preferable, and often required, a component failure should result in an open circuit. Transformer failures often result in a short circuit of the windings. Although the transformer inductance is made smaller, e.g., 0.25 Henry for the primary inductance and 1.0 Henry for the secondary inductance, the patented telecommunication line interface circuit still has several deficiencies. First, the prior art line interface circuit requires two transformers, one being about 2 cm.times.2 cm.times.1.78 cm, and the other about 1 cm.times.1 cm.times.1 cm, totalling about 8.1 cm.sup.3. Secondly, because the transformer primary windings are wound with the resistance-type conductors, a large part of the power loss due to the line feed current is generated internal to the transformer. The power dissipated is by way of heat, which is a mechanism that deteriorates transformers over time. Thirdly, because the current-limiting resistance of the transformer is internal thereto, voltage surges due to lightning appear directly across the transformer primary. The insulation between the windings must be adequate to withstand voltages caused by lightning strikes to the telephone line. In addition, the windings must be able to survive and dissipate the considerable surge energy without damage.
In U.S. Pat. No. 4,484,032 by Rosenbaum, an active impedance transformer assisted line feed circuit utilizes resistors connected to the transformer primary windings so that electrical surges due to lightning are effectively reduced before the energy reaches the transformer. Although the Rosenbaum line feed circuit utilizes a transformer for sensing line currents, it otherwise is a solid state line feed circuit that suffers from the headroom limitation noted above. The current sensing transformer facilitates common mode rejection and is used for AC impedance synthesis purposes.
U.S. Pat. No. 4,864,609 by Moisin discloses a telephone line interface circuit where a portion of the terminating impedance to the line constitutes resistors in the transformer primary, and the remaining terminating impedance resides in the electronic hybrid circuit. Despite that the primary of the transformer includes a portion of the terminating impedance, the inductance of the transformer primary winding is required to be about 1.7 Henry in order to provide a sufficient bandwidth at low voice frequencies. Moisin considers the resistors in the transformer primary circuit to be disadvantageous for one purpose, but helpful for another purpose, and thus utilizes impedance compensation techniques to effectively reduce the value of the line resistors.
U.S. Pat. No. 4,503,289 by Spires discloses a telephone line circuit having a resistor in the primary winding for sensing DC line currents. A compensation circuit requires an additional transformer winding for sensing the primary current and providing a compensated signal to the secondary winding to reduce the overall flux in the transformer core due to DC currents. An amplifier arrangement is provided for sensing voice signals across the tip and ring line, and coupling the signals to a transconductance amplifier to control the current delivered to the transformer secondary, thereby providing the proper AC terminating impedance for the tip and ring line. The Spires line feed circuit senses the line voltages and drives the transformer secondary with a current to synthesize a desired impedance. Because the transformer has a relatively large inductance (2 Henries), a voltage sense and current drive technique of impedance synthesis is acceptable.
Although the foregoing prior art illustrates the efforts to design telephone line feed circuits with smaller size transformers, additional advancements are possible in order to further reduce the size of the transformer component without sacrificing surge immunity and thereby allow a larger number of line interface circuits per unit of circuit board area. It can be seen that a need exists for further improvements in circuit design techniques for reducing the size of the transformer component, without compromising the other characteristics required, such as proper line terminating impedance, lightning protection, frequency response, line current capability, heat dissipation in the transformer, etc.